You need a scan tool to diagnose this. There are hundreds of possibilities for your running condition. A scan tool will provide you with the fault code to lead you to the component that is at fault. This could or could not be related to the valve cover you just had off, it could merely be a coincidence its happening now. Same with the oil light, its likely its a faulty sensor, but the fault code in the engine control module needs to be evaluated to determine this. I would recommend bringing the vehicle to a dealer or european specialty shop for a diagnostic fee. They will connect their scanner and give a printout of the fault codes and their diagnosis, then you can perform the repairs yourself if capable. I know this isn't what you are going to want to hear, but with the power train system on these vehicles, its impossible to guess what is causing your drivability concern.

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Thanks, JP